Methods for objectively evaluating the qualities of videos are classified into three types. That is, the first method is a method for directly comparing an original image and a degraded image to evaluate the quality of an image. The second method is a method for extracting only the characteristic amount of an original image and then evaluating the image quality based on the extracted characteristic amount and the degraded image. The third method is a method for evaluating an image quality based on only degraded images.
A specific example of the first method is recommended by ITU-T (International Telecommunication Union Telecommunication Standardization Sector), as described in non-patent document 1. However, the application for transmitting videos from a transmitter to receivers has an upper limit of a transmission line band, which makes it difficult transmit original images. If original images are not transmitted, the receiver cannot evaluate the image quality using the first method.
Moreover, the second method requires multiplexing the characteristic amount of an original image to original transmission image data and then transmitting the resultant data or transmitting the characteristic amount of an original image via another transmission line. When the characteristic amount of an original image is multiplexed and transmitted, the existing transmitter has to be replaced. Transmitting the characteristic amount of an original image via another transmission line requires replacing the existing communication system including the transmitter. In other words, the problem is that it is difficult to implement new operations in the existing system.
The third method has an advantage over merely requiring a replacement of the receiver because information on original images is not totally required, thus facilitating new operations. The image quality evaluating apparatus that estimates the image quality based on received data only, without using original images, is disclosed in the patent documents 1 and 2. In the image evaluating apparatus disclosed in the patent document 1, when received data is not correctly decoded due to, for example, an occurrence of transmission error on the transmission line, which transmits image data encoded for data amount compression, the number of regions where errors (errors in image data) have occurred is measured to determine it as a level of image quality degradation.
As described in the patent document 1, when image data cannot be decoded correctly, the decoding apparatus generally performs an error concealment process (an error hiding process), which obscures regions not correctly decoded, to video frames. By doing so, end users view images subjected to the error concealment process.    Patent document 1: Japanese patent Laid-open publication No. 2000-341588 (paragraphs 0044, 0072, 0095)    Patent document 2: Japanese patent Laid-open publication No. 2001-25014    Non-patent document 1: ITU-T recommendation J.144, March, 2004